The invention relates generally to the field of instruments for measuring and analyzing the optical properties of organic materials and, more particularly, to improved systems for measuring and analyzing optical densities of organic materials at various wavelengths to determine the percentages of certain constituents of the test material.
During the past few years there has been a growing need in the agricultural community for a versatle test instrument capable of rapidly determining moisture, oil and protein content in produce and grain products. The traditional analitical labratory techniques, for example the Kjeldahl technique for measuring protein, are extremely accurate but require the use of skilled chemist, and the results are not readily available. Buyers of agricultural products have manifested increasing interest in obtaining data on the percentage of moisture, protein and oil contained in various products. In fact, the export market, particularly in wheat, has seen the widespread introduction of selling on the basis of guaranteed protein content. This competitive pressure increases the requirement for the commodity handler, from the country elevator to the export terminal, to be able to rapidly and accurately sort grains and other products by their protein percentage and oil and water content, where applicable. Specifically, the need is for a versatile, yet low cost piece of advanced equipment which combines and improves upon recent scientific findings in the field of non-destructive testing of agricultural products. For maximum usefulness of commodity handlers, the instrument must not place high demands on the skillfulness of the operator or require a specialized knowledge of the scientific basis for an end result.
Prior researchers have confirmed that non-destructive light transmittance and reflectance tests on various agricultural products can be indicative of their moisture content and other characteristics. It has been demonstrated that the amount of light reflected at certain wavelengths from a uniform specimen of grain, for example, is indicative of moisture, protein and oil content. A most valuable measurement in this regard has been found to be the difference in optical density, delta OD, at two characteristic wavelengths. Optical density, OD, refers to the ease with which light is transmitted through or reflected by an object. Reflective optical density, OD, is defined herein by the equation, OD = Log (1/R), where R, reflectivity, equals the ratio of the intensity of reflected light to the intensity of incident light at a particular wavelength, I.sub.r /I.sub.i. Thus, delta OD = Log (I.sub.i /I.sub.r).sub.1 - Log (I.sub.i /I.sub.r).sub.2, where the subscripts 1 and 2 indicate the two different wavelengths used for the measurement of delta OD. If the intensity of incident light at both wavelengths is approximately the same, then delta OD = (I.sub.r).sub.2 - Log (I.sub.r).sub.1. Thus, it is known that by subtracting the two logarythms of the intensity of reflected light, an indication of the difference in reflective optical density can be obtained.
Research by the U.S. Department of Agriculture and other governmental agencies has indicated that individual .DELTA.OD readings selected to measure water, oil and protein content are interrelated. The following three equations have emerged from these studies as reliable indicators of the percentage of these constituents in agricultural products: EQU oil % = K.sub.O + K.sub.1 (.DELTA.OD).sub.w + K.sub.2 (.DELTA.OD).sub.o + K.sub.3 (.DELTA.OD).sub.p, (1) EQU water % = K.sub.4 + K.sub.5 (.DELTA.OD).sub.w + K.sub.6 (.DELTA.OD).sub.o + K.sub.7 (.DELTA.OD).sub.p, (2) EQU protein % = K.sub.8 + K.sub.9 (.DELTA.OD).sub.w + K.sub.10 (.DELTA.OD).sub.o + K.sub.11 (.DELTA.OD).sub.p, (3)
where K.sub.0 to K.sub.11 are constant coefficients or "influence factors" and the subscripts w, o and p indicate that the wavelengths at which the associated .DELTA.OD measurements are made are selected for their sensitivity to water, oil and protein content respectively. Before calculating the percentage constituents from the above equations, three different .DELTA.OD measurements must be performed on the specimen. Each .DELTA.OD requires measurements at two different wavelengths. Accordingly, six different wavelengths must be available. An instrument with the versatility to perform measurements on different agricultural products must be capable of switching from one set of six wavelengths to another set characterizing the optical properties of the product being tested.
Prior art devices for measuring .DELTA.OD employed a plurality of filters passing discrete nonvariable wavelengths. In the copending application Ser. No. 234,843, now U.S. Pat. No. 3,765,775, a specimen is illuminated with light sequentially filtered by a continuously rotating disc carrying a plurality of narrow band optical interference filters. The combined output of several photocells positioned to receive light reflected from the specimen is selectively sampled after passing through the logarythmic amplifier to obtain .DELTA.OD measurements. .DELTA.OD readings at two discrete wavelengths are compared in a differential amplifier to provide a .DELTA.OD measurement. Although this system is satisfactory for its intended purpose, the ability to make readings at various wavelengths is naturally limited by the number of filters carried by the disc. Without substituting a different filter, it is not possible to take readings at wavelengths between those of two filters having adjacent wavelengths.
A new optical filtering approach has recently been studied at the U.S.D.A. Agricultural Research Service in Greenbelt, Maryland. An ordinary narrow band with optical filter arrangement includes a planar interference filter inserted perpendicularly to the optical path between a wideband light source and the specimen. It has been found that by tilting the filter several degrees off the perpendicular orientation, the bandwidth may be shifted substantially, due to the difference in the apparent thickness of the filter as "seen" by the light source when the filter is tilted. If the filter is mounted for rotation through the optical path, the output of the filter is a continuously changing characteristic frequency over a limited range.
This system has been applied to .DELTA.OD measurements of agricultural products. Each pair of discrete filters for a given .DELTA.OD measurement, may be replaced by a single tilting filter, provided that the two frequencies of interest are close enough together. Of course, one of the essential advantages is that any frequency in the range "swept" by the tilting filter can be selected as desired, so that various OD measurements may be carried out on different specimens without changing the filter. One of the problems solved by the invention is incorporating the tilting filter technique in a fully automatic test instrument for use by unskilled operators.
A related problem arises in amplifying the output of the photocells. Because of the extreme sensitivity of photocells in current use, such as the lead sulphide type, the response of the photocell to light must be regularly compared or referenced to its response to darkness. In the past, rotating partial discs, called light choppers, have been used to provide alternating periods of darkness and illumination. The photocell's alternating output was capacitively coupled to a high gain amplifier and later demodulated to provide a DC (direct current) level representing the difference between the light and dark output. Besides requiring a very high voltage DC source before the amplifier, the demodulating technique superimposed noise on the photocell signal.